page.title=Implementing a Custom Request trainingnavtop=true @jd:body

This lesson teaches you to

Write a Custom Request

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Volley: Easy, Fast Networking for Android

This lesson describes how to implement your own custom request types, for types that don't have out-of-the-box Volley support.

Write a Custom Request

Most requests have ready-to-use implementations in the toolbox; if your response is a string, image, or JSON, you probably won't need to implement a custom {@code Request}.

For cases where you do need to implement a custom request, this is all you need to do:

Extend the {@code Request<T>} class, where {@code <T>} represents the type of parsed response the request expects. So if your parsed response is a string, for example, create your custom request by extending {@code Request<String>}. See the Volley toolbox classes {@code StringRequest} and {@code ImageRequest} for examples of extending {@code Request<T>}.
Implement the abstract methods {@code parseNetworkResponse()} and {@code deliverResponse()}, described in more detail below.

parseNetworkResponse

A {@code Response} encapsulates a parsed response for delivery, for a given type (such as string, image, or JSON). Here is a sample implementation of {@code parseNetworkResponse()}:

@Override
protected Response<T> parseNetworkResponse(
        NetworkResponse response) {
    try {
        String json = new String(response.data,
        HttpHeaderParser.parseCharset(response.headers));
    return Response.success(gson.fromJson(json, clazz),
    HttpHeaderParser.parseCacheHeaders(response));
    }
    // handle errors
...
}

Note the following:

{@code parseNetworkResponse()} takes as its parameter a {@code NetworkResponse}, which contains the response payload as a byte[], HTTP status code, and response headers.
Your implementation must return a {@code Response<T>}, which contains your typed response object and cache metadata or an error, such as in the case of a parse failure.

If your protocol has non-standard cache semantics, you can build a {@code Cache.Entry} yourself, but most requests are fine with something like this:

return Response.success(myDecodedObject,
        HttpHeaderParser.parseCacheHeaders(response));

Volley calls {@code parseNetworkResponse()} from a worker thread. This ensures that expensive parsing operations, such as decoding a JPEG into a Bitmap, don't block the UI thread.

deliverResponse

Volley calls you back on the main thread with the object you returned in {@code parseNetworkResponse()}. Most requests invoke a callback interface here, for example:

protected void deliverResponse(T response) {
        listener.onResponse(response);

Example: GsonRequest

Gson is a library for converting Java objects to and from JSON using reflection. You can define Java objects that have the same names as their corresponding JSON keys, pass Gson the class object, and Gson will fill in the fields for you. Here's a complete implementation of a Volley request that uses Gson for parsing:

public class GsonRequest<T> extends Request<T> {
    private final Gson gson = new Gson();
    private final Class<T> clazz;
    private final Map<String, String> headers;
    private final Listener<T> listener;

    /**
     * Make a GET request and return a parsed object from JSON.
     *
     * @param url URL of the request to make
     * @param clazz Relevant class object, for Gson's reflection
     * @param headers Map of request headers
     */
    public GsonRequest(String url, Class<T> clazz, Map<String, String> headers,
            Listener<T> listener, ErrorListener errorListener) {
        super(Method.GET, url, errorListener);
        this.clazz = clazz;
        this.headers = headers;
        this.listener = listener;
    }

    @Override
    public Map<String, String> getHeaders() throws AuthFailureError {
        return headers != null ? headers : super.getHeaders();
    }

    @Override
    protected void deliverResponse(T response) {
        listener.onResponse(response);
    }

    @Override
    protected Response<T> parseNetworkResponse(NetworkResponse response) {
        try {
            String json = new String(
                    response.data,
                    HttpHeaderParser.parseCharset(response.headers));
            return Response.success(
                    gson.fromJson(json, clazz),
                    HttpHeaderParser.parseCacheHeaders(response));
        } catch (UnsupportedEncodingException e) {
            return Response.error(new ParseError(e));
        } catch (JsonSyntaxException e) {
            return Response.error(new ParseError(e));
        }
    }
}

Volley provides ready-to-use {@code JsonArrayRequest} and {@code JsonArrayObject} classes if you prefer to take that approach. See Using Standard Request Types for more information.